Current methods for obtaining measured values for aspects of opaque thin films such as the thickness require a priori knowledge of one or more material properties of the thin film being measured. For example, a four-point probe measurement requires knowing the bulk resistivity of materials used in the film being measured to accurately calculate the thickness of the thin film. With this technique, four-point probe system is used to apply a current to an opaque film using two outer probes and simultaneously read the voltage drop across a portion of the film using the inner two probes. The current applied and the voltage measured together with the known bulk resistivity is used to calculate the thickness of the film. The calculation is simple, and described in detail in standard textbooks on semiconductor processing.
Though nominal values for materials currently being used in semiconductor applications are well known, these values are subject to variations in source materials used, variations in the process control parameters of the deposition tool used to form the film, etc.
U.S. Pat. No. 6,407,546, “Non-contact Technique for using an Eddy Current Probe for Measuring the Thickness of Metal Layers Disposed on Semiconductor Wafer Products,” issued Jun. 18, 2002 to Le et al., describes another type of technique used for determining metal film thickness is the eddy current probe. This can be used to measure a metal's thickness as well as the sheet resistance from a known resistivity constant. However, it requires a calibration sample i.e. a sample that covers a wide variety of thickness range to generate baseline measurements. This information is used while measuring on the inspection sample whose thickness and sheet resistance are unknown. At the beginning of the calibration process, the eddy current probe is placed in contact with the calibration sample. The measurements are usually performed on a test wafer. There are also methods based on optical reflectivity techniques.
U.S. Pat. No. 5,228,776, “Apparatus for evaluating thermal and electrical characteristics in a sample,” issued Jul. 20, 1993 to Smith et al., describes measuring changes in optical reflectivity caused by thermal waves to monitor variations in electrical conductivity and resistance as a method for evaluating the integrity of metal lines and vias in a semiconductor sample.
U.S. Pat. No. 6,054,868, “Apparatus and Method for Measuring a Property of a Layer in a Multilayered Structure,” issued Apr. 25, 2000 to Borden et al., describes using a thermal conductance method for determining sheet resistivity of a conductive layer or thermal conductivity of a dielectric layer located beneath the conductive layer. According to Borden et al. the invention relies on focusing a heating beam on a conductive layer and modulating its power at a frequency that is predetermined to be low enough such that the heat generated by the beam transfers out of the heated region only by conduction. The power of a second beam, called as the probe beam, whose phase is modulated with modulation of the heating beam, reflected by the heated region is measured.
A photoacoustic film thickness system such as the MetaPulse™ system, available from Rudolph Technology Incorporated, can be used to measure thin film thicknesses on wafers. Such a system forms two optical beams: an excitation beam used to excite the surface of the film sample periodically, and a probe beam used to sense the reflectivity of the surface of the sample following each pulse from the excitation beam. The time interval between excitation of the film surface and the measurement of the reflectivity is varied in a controlled manner to obtain a measurement of the surface reflectance as a function of time. This data is analyzed using the acoustical impedance of each layer in the thin film stack on the wafer and a software program to calculate the thickness of each layer in the film stack.
Examples of optical systems are provided in the following U.S. Patents.
U.S. Pat. No. 6,008,906, “Optical Method for the Characterization of the Electrical Properties of Semiconductors and Insulating Films,” issued Dec. 28, 1999 to Maris, describes a method for characterizing a sample that includes the steps of (a) providing a semiconductor material; (b) applying at least one of an electric field, a pulsed or cw light source, a change in temperature and/or a change in pump pulse intensity to the semiconductor material; (c) absorbing pump light pulses in a portion of the semiconductor material and measuring changes in optical constants as indicated by probe light pulses applied at some time t following the absorption of the pump light pulses; and (e) associating a measured change in the optical constants with at least one of a surface charge, dopant concentration, trap density, or minority carrier lifetime.
U.S. Pat. No. 4,710,030, “Optical Generator and Detector of Stress Pulses,” issued Dec. 1, 1987 to Tauc et al., describes an optical stress pulse generation and detection system for non-destructively measuring physical properties of a sample. This system uses a pump beam having short duration radiation pulses having an intensity and at least one wavelength selected to non-destructively generate a stress pulse in a sample. The system directs the non-destructive pump beam to a surface of the sample to generate the stress pulse. The optical stress pulse generation and detection system also uses a probe radiation beam and guides the probe beam to a location at the sample to intercept the stress pulse. The change in optical constants induced by the stress pulse is detected by observing the probe beam after it intercepts the stress pulse.
U.S. Pat. No. 5,379,109, “Method and Apparatus for Non-Destructively Measuring Local Resistivity of Semiconductors,” issued Jan. 3, 1995 to Gaskill et al., describes an apparatus for non-destructively measuring the resistivity of a semiconductor, such as InP. The system has light sources for illuminating a pre-selected portion of the semiconductor with first and second light beams, each of a pre-selected single wavelength. The first light beam operates to excite the semiconductor by photo-injecting carriers, and the second light beam bombards the local portion of the semiconductor with a pre-selected photon energy. The system measures a fractional change in reflectance of the second light beam responsive to the first light beam, and records this fractional change in reflectance for various values of photon energy of the second light beam, to generate a photoreflectance line-shape. The photoreflectance line-shape is used to calculate a photoreflectance line-shape phase angle, which is used to determine the resistivity of the pre-selected portion of the semiconductor.
U.S. Pat. No. 5,546,811, “Optical Measurements of Stress in Thin Film Materials,” issued Aug. 20, 1996 to Rogers et al., describes a method for determining the residual stress in an unsupported region of a thin film. The method includes the steps of (a) optically exciting the film with a spatially and temporally varying optical excitation field to launch counter-propagating acoustic modes along at least one wave vector; (b) diffracting a portion of an optical probe field off the excited acoustic modes to generate a time-dependent signal field at the excitation wave vector; (c) detecting the signal field to generate a time-dependent, light-induced signal; (d) analyzing the light-induced signal to determine the frequencies of the acoustic modes; (e) partially determining the dispersion of at least one mode; and, (f) comparing the measured dispersion to that calculated using a mathematical model to allow the residual stress properties of the unsupported region of the film to be determined.
U.S. Pat. No. 5,693,938 “Optical Probe Microscope Having A Fiber Optic Tip That Receives Both A Dither Motion And A Scanning Motion, For Nondestructive Metrology Of Large Sample Surfaces,” issued Dec. 2, 1997 to Marchman et al., describes an optical probe microscope that includes an optical fiber oriented in a vertical direction. The fiber has a tip that emits light onto a horizontal surface of a sample to be measured. This surface can have both desired and undesired departures from planarity. An electromechanical device for imparting dither motion to the fiber tip is superposed on another electromechanical device for imparting two-dimensional horizontal scanning motion to the fiber tip. The dither motion has a much higher frequency than that of the scanning motion. Between successive scans, another device moves the sample itself from one horizontal position to another. A microscope receives the optical radiation either transmitted or reflected by the sample surface. The microscope forms a (magnified) image of this received optical radiation on the surface of an optical image position detector. The surface of this detector has a relatively large area compared with that of the (magnified) image. The resulting electrical signal developed by the detector provides desired information concerning the scanning position of the fiber tip. Also, this electrical signal is processed and fed back to a vertical pusher that maintains constant the distance of the fiber tip from the sample surface.
U.S. Pat. No. 6,038,026, “Apparatus and Method for the Determination of Grain Size in Thin Films,” issued Mar. 14, 2000 to Maris, describes a method for the determination of grain size in a thin film sample having steps of measuring first and second changes in the optical response of the thin film, comparing the first and second changes to find the attenuation of a propagating disturbance in the film and associating the attenuation of the disturbance to the grain size of the film. The second change in optical response is time delayed from the first change in optical response.
U.S. Pat. No. 5,959,735, “Optical Stress Generator and Detector,” issued Sep. 28, 1999 to Maris et al., describes a system for the characterization of thin films, as well as interfaces between thin films, through measurements of their mechanical and thermal properties. In the system light is absorbed in a thin film or in a structure made up of several thin films, and the change in optical transmission or reflection is measured and analyzed. The change in reflection or transmission is used to give information about the ultrasonic waves that are produced in the structure. The information that is obtained can include (a) determination of the thickness of thin films with a speed and accuracy that is improved compared to earlier methods; (b) a determination of the thermal, elastic, and optical properties of thin films; (c) a determination of the stress in thin films; and (d) a characterization of the properties of interfaces, including the presence of roughness and defects.
U.S. Pat. No. 5,844,684, “Optical Method for Determining the Mechanical Properties of a Material,” issued Dec. 1, 1998 to Maris et al., describes a system and method for characterizing a sample. The method includes steps of (a) acquiring data from the sample using at least one probe beam wavelength to measure, for times less than a few nanoseconds, a change in the reflectivity of the sample induced by a pump beam; (b) analyzing the data to determine at least one material property by comparing a background signal component of the data with data obtained for a similar delay time range from one or more samples prepared under conditions known to give rise to certain physical and chemical material properties; and (c) analyzing a component of the measured time dependent reflectivity caused by ultrasonic waves generated by the pump beam using the at least one determined material property. The first step of analyzing may include a step of interpolating between reference samples to obtain an intermediate set of material properties. The material properties may include sound velocity, density, and optical constants. In one embodiment, only a correlation is made with the background signal, and at least one of the structural phase, grain orientation, and stoichiometry is determined.
Though nominal values for materials currently being used in semiconductor applications are well known, these values are subject to variations in source materials used, variations in the process control parameters of the deposition tool used to form the film, etc.
Although the four-point probe system and photoacoustic film thickness systems provide a technique for measuring the thickness of a material, it would be advantageous to be able to verify test information as a means to insure accuracy.